Maaser Sheni 3:2-3

One may not purchase terumah with second tithe money, because that reduces the number of people who may eat it; Rabbi Shimon allows it. Rabbi Shimon said: If we are lenient in the case of peace offerings, which can be rendered unfit by improper intentions, by being left too long, or through ritual impurity, shouldn’t we also be lenient regarding terumah? The Sages replied: If we are lenient in the case of peace offerings, which may be eaten by non-kohanim, does that mean we should be lenient regarding terumah, which may not be eaten by non-kohanim?

Maaser Sheini 3:3

If one has second tithe money in Jerusalem and he needs it for other purposes, if his friend has produce, he may tell the friend, “This money is made secular by transferring its sanctity onto your produce.” The owner of the produce must then eat it in a state of ritual purity and the other may use the money for any purpose. One may not do this with an unlearned person (who is not reliable regarding tithes and ritual purity) except in the case of demai (doubtfully tithed produce).